The present invention relates to an intersomatic implant intended in particular for holding a bone graft stationary between two vertebrae, and more particularly to a cage suitable for containing grafts.
The fields of application of the invention lie in particular in surgical treatment of degenerative disks.
In general, such treatment involves an operation seeking to lock two adjacent vertebrae together, with this being done by interposing bone grafts between them so that once the graft has taken the vertebrae are bonded together. However, given the mechanical stresses exerted on the grafts by the two vertebrae, there is a danger of the grafts being expelled or crushed.
To mitigate that drawback, it is known to hold the grafts in place by means of cages whose top and bottom ends bear against the vertebrae via edges around openings that enable the grafts to make contact with the superior and inferior vertebral plates.
Locking two vertebrae together by means of an element of bone presents the advantage of imparting uniform mechanical properties to the resulting assembly, and in particular uniform elastic deformation of said assembly as characterized by its modulus of elasticity.
Unfortunately, the cages in general use are made of titanium alloy and the modulus of elasticity of such alloys are at least ten times greater than the modulus of elasticity of vertebral bone. Thus, intersomatic cages that remain permanently between vertebrae, themselves give rise to discontinuity in the mechanical properties of the assembly made up of two bonded-together vertebrae.
In order to conserve uniform elastic deformation, proposals have been made to use cages made of macromolecular materials having moduli of elasticity that are of the same order of magnitude as those of bone. However, plastics material cages which are generally annular in shape are not as strong as titanium alloy cages and inserting them by force between vertebrae can lead to their edges rupturing. The solution which consists in enlarging their edges reduces the volume available for grafts and thus the bonding force between two vertebrae. Furthermore, since they are not as hard as titanium alloy cages, their edges penetrate to a smaller extent into the vertebral plates, so they are not prevented so effectively from moving relative to the vertebrae.
An object of the present invention is to provide an intersomatic implant made of macromolecular materials, the implant being of the cage type, presenting top and bottom openings of the same order of magnitude as the openings in titanium alloy cages so as to retain a graft area of substantially equal size and thus ensure that the graft is held in a fixed position relative to the vertebrae.
To this end, the present invention provides an intersomatic implant suitable for containing at least one bone graft and for being inserted in the intervertebral space along a predetermined direction, the implant comprising: a first portion of generally annular shape surrounding an aperture and presenting a top first edge and a bottom first edge; and a second portion of elongate shape along its main axis presenting a bottom second edge and a top second edge, and interconnecting two facing opposite regions of said first portion along said predetermined direction, thereby dividing said aperture into two housings suitable for containing said bone graft, at least one of said bottom and top second edges projecting from the space defined by said bottom and top first edges of the first portion.
Thus, a characteristic of the intersomatic implant lies in said generally annular first portion being reinforced by an elongate second portion situated in the determined direction along which said implant is pushed in between the vertebrae. As a result, in spite of the stresses that are exerted on the first portion while it is being impacted between the vertebrae, which stresses tend to deform the first portion in its midplane so as to move the two substantially opposite regions towards each other, said first portion retains its generally annular shape. In addition, the top and bottom open areas, subdivided into two by said second portion, are of the same order of magnitude as the areas of a titanium alloy intersomatic implant, thus making it possible to obtain a graft area that is substantially identical.
In addition, the bottom and top second edges project out from the space defined by said bottom and top first edges of the first portion, such that not only is the implant held between the vertebrae by contact between the edges of said first portion against the vertebral plates, but it is also held by said elongate second portion which bears against the vertebral plates which are of biconcave shape.
The implant of the invention is prevented from moving between the vertebrae in a manner that is more reliable than with a conventional implant that is of annular structure, only.
In a first embodiment of the invention, said first portion and said second portion are connected together so as to form a single piece, ensuring that they are entirely cohesive.
Advantageously, said bottom and top second edges of said second portion are curvilinear in shape with their ends running into said bottom and top first edges respectively of said first portion. As a result, the shape of the second portion coincides substantially with the shape of the available space situated between the vertebrae.
In a preferred embodiment of the invention, said first portion is pierced laterally along said main axis of said second portion so as to provide a blind hole extending in said second portion, said blind hole being suitable for receiving an insertion tool.
As explained below in greater detail, in this preferred embodiment, the implant is inserted between two adjacent vertebrae by force and it is necessary to have complete control over the insertion direction. In order to conserve a constant angle of impact, the tool which pushes said implant is itself inserted into a blind hole which extends in the second portion. Thus, the end of the tool pressed against the implant is held captive in said implant during impacting and does not run any risk of puncturing tissue situated in the vicinity of the operation.
Advantageously, said bottom and top edges of said first and second portions present serrations suitable for constituting anchoring points in said vertebrae.
Thus, the sharp portions of the serrations penetrate into the walls of the vertebral plates. As a result, the implant is held between the vertebrae without any possibility of moving laterally, thus encouraging the grafts to take.
Preferably, said first portion of generally annular shape has at least one axis of symmetry and advantageously, said second portion interconnects said two opposite regions of said portion along said axis of symmetry.
This configuration makes it possible to match the implant accurately to the shape of the vertebrae, and in particular of the vertebral bodies. Since said second portion lies on the axis of symmetry, the stresses exerted on the implant during impacting are distributed uniformly.
In another preferred embodiment of the invention, said second portion interconnects said two opposite regions of said first portion along a direction that is at an angle lying in the range 0xc2x0 to 90xc2x0 with said axis of symmetry.
Thus, the implant can be inserted between the vertebrae more easily when the insertion is performed does not make it possible to operate symmetrically about the spinal column. It is always necessary to push the implant in along the axis of said second portion in order to avoid breaking said first portion, and at certain levels, the vertebrae are accessible laterally only. Under such circumstances, said second portion is disposed obliquely relative to the axis of symmetry of said implant.
Preferably, said first portion is generally substantially semicircular in shape.
In another preferred embodiment of the invention, said part is molded out of a macromolecular material. Thus, it can be made under economically advantageous conditions. Said part is preferably made of polyether ether ketone which is easy to mold and which presents elastic properties close to those of bone.
In a second embodiment of the invention, said second portion further comprises anchoring means projecting from said top and bottom second edges, respectively. Thus, said anchoring means are caused to penetrate by force into the surfaces of the vertebral plates while the implant is being inserted between the vertebrae, thereby ensuring that said implant is completely prevented from moving in translation relative to the vertebrae and cannot slide relative thereto.
In a particular embodiment, said anchoring means are constituted by an anchoring piece having two ends, and said second portion has a slot opening out into said bottom and top second edges so that said anchoring piece when inserted in said slot passes through said second portion, the first end projecting from said top second edge and the second end projecting from said bottom second edge of said second portion.
As a result, the anchoring piece can be inserted in the second portion at the time it is inserted between the vertebrae, should that be absolutely necessary. In addition, since the anchoring means are made as a single piece, their rigidity is increased and they do not deform during insertion.
Advantageously, said anchoring piece is substantially trapezoidal in shape, its two ends being defined by the two non-consecutive and non-parallel sides thereof. Preferably, it has a midplane, and it is inserted in said second portion so that said main axis A of said second portion intersects the two bases of the trapezoid, and its midplane is substantially perpendicular to the midplane of said first portion. Thus, by means of the aperture which is substantially in the form of a rectangular parallelepiped, the walls of the anchoring piece coincide with the walls of the aperture.
Preferably, one of the bases of the trapezoid forms two acute angles with the two non-parallel sides, and said anchoring piece is inserted in said second portion so that the insertion direction of said implant extends from the longer base towards the shorter base of the trapezoid. As a result, the anchoring piece presents a projecting point at each end, defined by the acute angle and suitable for penetrating more deeply into the vertebral body so as to be secured even more firmly thereto. In addition, the anchoring piece forms an arrow portion when it is engaged between the vertebrae, when the implant is pushed in from the larger base of the trapezoid towards the smaller base. As a result, it is impossible for the anchor piece to move in the opposite direction due to the projecting point penetrating into the vertebral plates. Preferably, the anchoring piece is made of titanium alloy, and as a result its rigidity enables better anchoring to be obtained.
The present invention also provides an intersomatic implant press for inserting an anchoring piece in the aperture of said second portion. The press of the invention comprises: a first jaw suitable for receiving said first portion in which said second portion interconnects two opposite edges, said first jaw having a central aperture in register with said slot in said second portion; and a second jaw placed facing said central slot; and said jaws are capable of being moved towards each other in such a manner that said second jaw can bear against said anchoring piece to force it into said slot or to extract it from said slot.